Limestone (CaCO.sub.3) or hydrated lime (Ca(OH).sub.2) used in pulverized or fluidized bed combustors for SO.sub.2 removal suffer from low reactivity and under utilization. In spite of being economical and easily retrofittable in the existing utility units, dry sorbent processes fail to be more competitive with other more expensive SO.sub.2 control technologies due to their poor SO.sub.2 removal efficiency and low sorbent utilization. Typically, less than 50% of the available calcium is converted to high molar volume calcium sulfate product which causes pore blocking and pore mouth plugging and renders the sorbent ineffective for any further SO.sub.2 capture. The spent sorbent from pulverized combustors (PC's) exhibits less than 35% calcium utilization, while for circulating fluidized bed combustors (CFB's), up to 45% calcium utilization is realized (Couturier et al., 1994).
The spent sorbent exhibits negligible reactivity towards SO.sub.2 and in order to increase the sorbent utilization, the sorbent needs to be reactivated to expose the unreacted CaO. Reactivation of the under utilized sorbent would necessarily require, re-exposing and/or redistribution of the CaO from the interior of the sorbent particle and reactivation of the sintered CaO by converting it into a more reactive form. The fundamental challenge and goal of the reactivation process is to redistribute the CaSO.sub.4 predominantly from the surface of the particle to a more uniform distribution.
One of the methods for reactivating partially utilized sorbents is by the process of hydration (Bobman et al., 1985). In this process, the unsulfated CaO is reacted with water to form Ca(OH).sub.2. Due to higher molar volume of the hydroxide (33 cc/gmol), compared to CaO (17 cc/gmol), the sorbent particles expands and the non-porous CaSO.sub.4 shell cracks thereby exposing the hydrate. Moreover, once this reactivated sorbent is reintroduced into the combustor, calcination of Ca(OH).sub.2 further increases the porosity and provides added exposure of CaO to SO.sub.2. Hydration has been known to increase the utilization of spent sorbent from 35% to up to 70% (Couturier et al., 1994). It is known that the effectiveness of the hydration reactivation process is dictated by the duration of hydration, the hydration temperature, and the solids concentration in the process. The temperature for drying the hydration products has also been indicated to markedly affect the activity of the reactivated product (Khan et al., 1995; Tsuchia et al., 1995).
Researchers (Al-Shawabkeh et al., 1997) have studied the slurry-based hydration (3% solids concentration) of calcined dolomitic particles to produce an effective sorbent for SO.sub.2 removal. They observed a 1.3-1.6 fold increase in SO.sub.2 capture ability at 900.degree. C. in a thermo-gravimetric setup. In their study, increasing hydration time and temperature had a favorable effect on the hydration activation. Several researchers have reported that a recycle of spent sorbent and fly ash mixture into the spray dryer results in substantial improvements in reagent utilization and SO.sub.2 removal (Melia et al., 1983; Palazzolo et al., 1984; Parson et al., 1981). It has been suggested that substantial reactions take place between the fresh Ca(OH).sub.2 and recycled fly ash from spray dryer resulting in the formation of hydrated calcium silicates and their subsequent reaction with SO.sub.2 leads to the increased efficiencies. Laboratory experiments conducted with slurried Ca(OH).sub.2 and one of the fly ash components show several folds increase in sorbent utilization (Josewicz and Rochelle, 1986) From these results they concluded that enhanced utilization of the recycle fly ash and calcium solids is probably due to reaction between Ca(OH).sub.2 and fly ash to produce calcium silicates which have greater surface area than unreacted Ca(OH).sub.2 and are more effective for gas-solid reactions. Researchers have also suggested that increased time and temperature of the slurrying process gives more reactive solids.
Characterization and testing of hydrated mixtures of fly ash and Ca(OH).sub.2 under laboratory conditions suggests that hydrated mixtures develop higher total surface area than the arithmetic sum of surface areas of initial solids before hydration (Martinez et al., 1991). These studies also report that the incremental surface area increased with temperature, time of hydration, and fly ash/Ca(OH).sub.2 ratio, with temperature effect being the strongest. Tetracalcium aluminate monosulfate and tetracalcium aluminate are assumed to be responsible for the increased surface areas. Removal efficiencies of up to 65% are reported (Hurst et al. 1981), with a slurry of highly alkaline (20% unutilized CaO) fly ash only.
Josewicz et al. (1987) investigated the reactivation of boiler limestone injected solids via hydration process for enhancing their calcium utilization. They have reported that the activity of sorbents prepared from spent furnace sorbent (containing CaO and CaSO.sub.4), and fly ash, is greatly influenced by the hydration reaction. Their studies also report that the activity of spent sorbent increased with the hydration time, which resulted in the formation of ettringite and calcium silicate. Josewicz et al. (1987) have cited that the main factor for enhancement of SO.sub.2 capture is the pozzolonic reaction on combining recycled solids and water.
Reactivation of spent limestone samples from circulating fluidized bed combustor via hydration has been found to cause particle expansion with increase in their internal volume (Couturier et al., 1994; Shearer et al., 1980; Marquis, 1992). Couturier et al. (1994) determined that the conversion of available calcium to CaSO.sub.4, in the treated/reactivated bed particles increased from 32% to 80%. They suggested that hydration creates new pores and internal volume of the particle. The water permeates through the partially sulfated layer and reacts with the inner CaO core to form calcium hydroxide. The hydroxide having the higher molar volume swells and cracks the partially sulfated shell. Marquis (1992) studied the correlation between the extent of conversion of CaO in fly ash to Ca(OH).sub.2 during hydration and the utilization of Ca upon re-sulfation and observed that with increasing conversion of CaO to Ca(OH).sub.2 the extent of calcium utilization increased upon sulfating the reactivated sorbent.
The above mentioned mechanisms for reactivation of spent sorbent via hydration suggest that big particles (greater than 200 mm, the typical ash particle size obtained from fluidized bed combustors), undergo reactivation by particle expansion and subsequently develop cracks on the outer inactive sulfate shell (Couturier et al., 1994). Reactivation of particles that are of smaller dimension, (such as from the bag-house), might be due to reactions between silica/alumina species and calcium leading to the formation of Ca--Si--Al complexes. These complexes have high surface areas and are highly effective for gas-solid reactions (Josewicz and Rochelle, 1986).
As suggested by several researchers (Ghosh-Dastidar et al., 1996; Mahuli et al., 1997; Gullett and Bruce, 1987), the key to the high reactivity of a sorbent, fresh or partially utilized, lies in its open initial internal structure and subsequent pore structure evolution under high temperature conditions.
Accordingly, there remains a need for a method of efficiently recycling calcium-based sorbents.
It is also an object of the present invention to be able to provide recycled calcium-based sorbents that are able to perform substantially as new sorbents, to prevent waste and thereby achieve more complete usage of calcium-based sorbents.
In addition to the features mentioned above, objects and advantages of the present invention will be readily apparent upon a reading of the following description.